The present invention is directed to polarization independent optical filters.
Optical signal transmission encounters problems similar to those found in electrical networks, and also encounters problems unique to optical networks. For example, both electrical and optical networks must handle ever-increasing amounts of data (e.g., voice, video, audio, text, graphics, etc.). For optical networks, various multiplexing schemes are employed (e.g., wave division multiplexing (WDM), dense WDM, and ultra-dense WDM) to increase transmission bandwidth by simultaneously transmitting data from a plurality of sources to a plurality of destinations over a single optical medium such as, for example, a fiber-optic cable or waveguide. Obviously, the data from the plurality of sources is not intended for the same destination and it is necessary to selectively switch and route the various data to its intended destination using filters, switches, couplers, routers, etc.
An optical signal typically comprises a plurality of wavelengths, with each wavelength representing data from a different source. An optical network must be able to direct each wavelength (i.e., each separate data source), separate from the other wavelengths, over various paths in the network. Switching/filtering not only facilitates routing of a desired wavelength to its intended destination, it also facilitates re-routing in the case of network failure (e.g., fiber-optical cable breakage), or to alleviate network congestion, as two examples. As the need for bandwidth continues to increase (whether for optical or electrical networks), so too does the need to distinguish the various signals being simultaneously transmitted.
Unique to optical transmission are the polarization modes of an optical signal. A single optical signal may have both transverse electric (TE) and transverse magnetic (TM) modes, each propagating through the optical components of the network at different speeds and generally experiencing slightly different conditions. A WDM signal, for example, is randomly polarized, with each wavelength having a different, independent polarization. Optical fiber, for example, has a small birefringence such that, after propagating through any substantial length of fiber, the optical signal arriving at the end of the fiber will have a random, unpredictable polarization different from that at the input end of the fiber. Thus, any subsequent optical component into which the optical signal is coupled must not differentiate between the different polarizations if the optical network is to be transparent to polarization. It is thus desirable to provide an optical system, component, and/or device that is essentially polarization independent and that enables transmission of a randomly polarized multi-wavelength optical signal.
It is desirable that optical networks (and the systems and components that make up the networks) be capable of handling both polarization modes.
It is thus desirable to provide an optical a system, component, and/or device that is essentially polarization independent and that enables transmission of a polarized multi-wavelength optical signal.
The present invention is directed to a polarization independent, channel-dropping optical filter comprised of two complementary filtering elements: one tuned for TE polarization mode and the other for TM polarization mode. The filtering elements arc preferably tuned to the same predetermined wavelength so that a specific wavelength may be separated (i.e., filtered) from a randomly polarized wavelength division multiplexed (WDM) optical signal. The filtering elements also preferably have the same peak transmission characteristics. The present invention also advantageously filters both polarization modes of the desired wavelength. The filter further comprises input and output waveguides which strongly confine and guide a polarized multi-wavelength optical signal. Each waveguide is separated from the filtering elements by a gap over which the optical signal may be coupled to and from the filtering elements.
The filtering elements preferably comprise a micro-ring or circular disk resonator, or a non-circular micro-ring resonator with substantially straight sections that define a coupler length that facilitates light transfer between the waveguides and resonators. The resonators of the inventive filter preferably satisfy the following requirements: one resonator couples only TE polarization mode and the other couples only TM polarization mode; the resonators are tuned to the same resonance wavelengths for both TE and TM polarization modes; and the resonators have the same transfer characteristics for both TE and TM polarization modes.
The resonators are preferably photonic-well or photonic-wire waveguides that strongly confine (e.g., a the planar direction) and guide light. The strong confinement characteristics make it possible to construct resonators having relatively small bend radii (e.g., on the order of approximately 10 microns).
The evanescent coupling between the straight waveguides and the resonator is dependent on the gap size, the waveguide width, and the material indices inside and outside the waveguides. For better control of the coupling, a race-track shaped resonator may be used, i.e., one with substantially straight coupling sections having a pre-determined length and that are disposed in substantially parallel relation with the input and output waveguides. With proper choice of the gap size and waveguide width, for example, it is possible to design the resonator to be favorable for either TE or TM modes. The exact coupling factor will then be determined by the length of the straight coupling section.
Preferably, the filter is constructed having the following parameters: a gap is defined between the input waveguide and resonators, and between the resonators and output waveguide that has a width which is less than 0.5 xcexcm; the width of the waveguides (including the waveguides of the resonators) is less than 1 xcexcm; the coupler length is less than 50 xcexcm; and the ratio of the index of refraction inside the waveguides to the index of refraction of the medium (e.g. air) in the gap between the waveguides is greater than 1.5.
The operation of the filter is affected by the polarization of the light signal. For TE mode signals, it is preferred that the width of the waveguides be less than 0.25 xcexcm. As for TM mode signals, it is preferred that the width of the waveguides be greater than 0.35 xcexcm.
The relationship between the waveguide width and gap width and their effect on polarization is discussed in detail in co-pending patent application Ser. No. 09/574,835 entitled Nanophotonic Directional Coupler Device, the entire content and disclosure of which is hereby incorporated by reference.
It is preferred that symmetry be achieved in the filter design and construction. Specifically, the waveguides are similarly or substantially similarly formed (e.g., materials, dimensioning, etc.) to enable efficient transfer of the light signal between the input/output waveguides and the resonators.
Thus, the present invention is directed to a novel optical filter comprised of polarization dependent components to provide a polarization independent device.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.